Stabilizing bandwidth, such as, E95 for DUV semiconductor integrated circuit photolithography laser light source design has included passive and active bandwidth controls. Active controls, can benefit such things as optical performance correction (“OPC”) and tool-to-tool matching for such light sources. U.S. Provisional Patent Application Ser. No. 60/923,486, entitled TWO STAGE EXCIMER LASER WITH SYSTEM FOR BANDWIDTH CONTROL, filed on Apr. 13, 2006, includes the disclosure of U.S. patent application Ser. No. 11/510,037, entitled ACTIVE SPECTRAL CONTROL OF DUV LIGHT SOURCE, filed on Aug. 25, 2006, published on Aug. 23, 2007, Pub. No. US-2007-0195836-A1, which claims priority to U.S. Provisional Application Ser. No. 60/774,770, entitled ACTIVE SPECTRAL CONTROL OF DUV LIGHT SOURCES FOR OPE MINIMIZATION, filed on Feb. 17, 2006, the disclosures of each of which are hereby incorporated by reference. application Ser. No. 11/510,037, discloses a multistage bandwidth control system using coarse and fine control actuators.
That application discloses the utilization of grating bending (the position of a bandwidth control device “BCD” which bends the grating) as an actuator (control authority) along with other actuators. Various techniques are discussed enabling bandwidth stabilization. Active bandwidth control systems can use very accurate on-board spectrum measurement, such as E95, and bandwidth error feedback. Compensation based on other laser parameter/output signals, such as target energy and duty cycle, can enable the control of various bandwidth selection actuators, including low frequency large amplitude actuators and a high frequency small amplitude actuators.
A photolithography light source laser, a Multiple-Input Multiple Output (MIMO) time varying, nonlinear system, could use an actuator(s) which itself could cause other effects to the laser performance than changing bandwidth, desirable or otherwise. Multi-stage, e.g., dual stage actuator designs, including affecting laser behavior with separate operating parameter inputs (actuators), working together, could be optimized to respond to a particular class(es) of disturbance(s). Disturbances can be categorized by the time scale and/or magnitude of impact. Pulse energy settings can include low magnitude fast time scale disturbances (typically msec to sec in a fine actuation range). High (timescales in seconds) and low (hours) frequency aspects of duty cycle set point and fluorine gas consumption (hours) changes can induce larger magnitude effects. Other longer-term parameter changes, component aging and misalignment (days to weeks or even longer) can result in the largest magnitude (in a coarse actuation range) changes.
Accordingly control action divided into coarse actuation and fine actuation was disclosed, with each utilizing one or more parameter change actuators. One could target large magnitude low frequency disturbances (large E95 setpoint changes, gas aging effects and the long timescale component of duty cycle changes—resulting, e.g., from slow thermal loading variations, increasing age of laser components and the like). The other could target smaller magnitude higher frequency disturbances (output pulse energy, and the fast component of duty cycle changes—resulting, e.g., from faster thermal loading transients and the like). The coarse actuator can also serve to de-saturate, or re-center, a fine actuator(s) within its control range.
Coarse actuators, such as, (F2 gas injection) and fine actuator (ΔtMOPA), grating bending or other wavefront conforming adjustments, or adjustable aperturing the beam, etc. were shown to regulate bandwidth, with various impacts on other laser parameters and with time frames of measurement for feedback and actuation allowing decoupling and use together for bandwidth control. The term dtMOPA or ΔtMOPA or differential firing timing, or differential discharge timing, or differential commutation control, as used herein, are all shorthand notations for the concept of timing the discharge between the seed laser electrodes and the amplifier laser electrodes so as to selectively amplify a portion of the seed laser pulse in the amplifier gain medium to select bandwidth of the output of the laser light source. Sufficient ranges of actuation were noted to be able to account for bandwidth deviation from such affects that commonly need to be suppressed as from long term duty cycle variations, gas aging and component aging. The controllers could be compensated for/desensitized to error signal variations due to other laser operating parameters (filtering and other normalization).
Variable magnification of a beam incident on a center wavelength selection optical element, such as a dispersive grating, can also affect bandwidth of the light source. Such a system is discussed in U.S. Pat. No. 6,393,037 which issued to Basting et al. on May 21, 2002, the contents of which are hereby incorporated by reference herein. The abstract of Basting describes a tunable laser including an angular dispersive optical element (grating) and a beam expander including one or two rotatable prisms in a line narrowing module to adjust the bandwidth. The prism beam expanders, when two are used, are disclosed to be mechanically linked to each other so that the angle of incidence of the beam on the dispersive element is not changed when the magnification changes. This arrangement makes it very difficult, if not impossible, to control center wavelength as well as bandwidth utilizing the rotatable prisms. In addition the bandwidth control system using the two prisms operating in tandem is quite difficult.
GigaPhoton is believed to advertise a product that performs E95 control using some sort of optical actuation. Japanese Published Patent Application 2006024855, published on Jul. 9, 2004, discloses a variable magnification LNM also, with two rotatable prisms and the use of a differential discharge firing timing between a seed laser and amplifier laser for bandwidth control. Such an arrangement makes it difficult, if not impossible, to control center wavelength and bandwidth using the prisms, and controlling bandwidth with dtMOPA in such a system may have certain shortcomings.
The existing types of active bandwidth control can suffer from a number of shortcomings, such as taking too long to recover from a large disturbance such as a transient after a fluorine injection, or from a change in target bandwidth. These could cause the bandwidth to go off target by as much as ≧5 fm, considering as an example a 55 shot moving average or ≧6 fm, considering as an example a 55 shot moving standard deviation, added in quadrature. A return to the target could take more than ten seconds.
In addition, especially at low duty cycles, a target change, such as from 300 fm to 400 can be too slow, i.e., taking as long as about twenty seconds or from 400 fm down to 290 fm taking as long as about thirty seconds, when the requirement may be as low as around 10 seconds.
Among other things, errors can result from a filtering of control signals, such as a holdoff of too many pulses before a controller step signal can be issued to a bandwidth actuator stepper, such that at low duty cycles where bandwidth may be sampled less frequently, there is a much higher chance that step commands will not occur during a burst. Since the stepper commands or the stepper itself may be disabled during an inter-burst interval, burst to burst errors in bandwidth can thus build upon each other, with no correction signals issued to the bandwidth control actuator(s).
Prior art bandwidth stability controls enabled the setting of a bandwidth control set point in a set and forget manner. More stringent requirements call for a tunable bandwidth setting controllable by the end user of the laser light source. It is also required that the bandwidth control system not negatively impact the control of other laser operating or output parameters, such as output pulse energy and dose stability and the like.